1. Field of the Invention
This invention relates to a synergistic combination comprising a polyether polyamino methylene phosphonate, a terpolymer comprising the monomers of acrylic acid, sulfophenomethallyl ether and maleic acid, and hydroxyphosphonoacetic acid for controlling the deposition of calcium carbonate and calcium phosphate scale deposits on the surfaces of an aqueous system.
2. Brief Description of the Background Art
Generally, calcium carbonate and calcium phosphate scale deposits are incrustation coatings which accumulate on the metallic or plastic surfaces of a water-carrying system through a number of different causes.
Various industrial and commercial water-carrying systems are subject to calcium carbonate and calcium phosphate scale formation problems. Calcium carbonate and calcium phosphate scales are of particular concern in heat exchange systems employing water, such as, for example, boiler systems and once-through and open recirculating water cooling systems. Cooling towers are especially significant, particularly where severe conditions including high pH and high calcite concentrations are encountered.
The water employed in these systems ordinarily will contain a number of dissolved salts, and the alkaline earth metal cation calcium is usually prevalent, as are the anions carbonate and phosphate. The combination products of calcium cation and carbonate anion and calcium cation and phosphate anion will precipitate from the water in which the ions are carried to form scale deposits when the concentrations of the anion and cation comprising the reaction product, i.e., calcium carbonate, or calcium phosphate, exceeds the solubility of the reaction product itself. Thus, when the concentration of calcium ion and anion exceed the solubility of the calcium reaction product, a solid phase of calcium carbonate and/or calcium phosphate will form as a precipitate. Precipitation of the reaction product will continue until the solubility product concentration of the constituent ions is no longer exceeded.
Numerous factors may be responsible for producing a condition of supersaturation for the reaction product. Among such factors are changes in the pH of the water system, evaporation of the water phase, rate of heat transfer, amount of dissolved solids, and changes in the temperature or pressure of the system.
For cooling systems and similar heat exchange systems, including cooling towers, the mechanism of scale formation is apparently one of crystallization of scale-forming salts from a solution which is locally supersaturated in the region adjacent the heating surface of the system. The thin viscous film of water in this region tends to become more concentrated than the remainder of the solution outside this region. Precipitation is also favored on the heat transfer surface because of the inverse solubility relationship of calcium carbonate. As a result, the solubility of the scale-forming calcium carbonate salt reaction product is first exceeded in this thin film, and crystallization of calcium carbonate scale results directly on the heating or heat exchange surface.
In addition to this, a common source of scale in boiler systems is the breakdown of calcium bicarbonate to form calcium carbonate, water and carbon dioxide under the influence of heat. For open recirculating cooling water systems, in which a cooling tower, spray pond, evaporative condenser, and the like serve to dissipate heat by evaporation of water, the chief factor which promotes calcium carbonate scale formation is concentration of solids dissolved in the water by repeated evaporation of portions of the water phase. Thus, even a water which is not scale forming on a once-through basis usually will become scale forming when concentrated two, four or six times. Moreover, alkalinity of the make-up water, with evaporative cycles over time results in an increasing alkalinity of the water in the overall system, often having pH's of 8.5-9.5 and even higher. Conventional scale inhibiting compositions typically fail in systems having such severe conditions.
The formation of calcium carbonate and calcium phosphate scale deposits poses a serious problem in a number of regards. The calcium scale which is formed possesses a low degree of heat conductivity. Thus, a calcium scale deposit is essentially an insulating layer imposed across the path of heat travel from whatever source to the water of the system. In the case of a cooling system, the retarded heat transfer causes a loss in cooling efficiency. Consequently, calcium scale is an expensive problem in many industrial water systems, causing delays and shutdowns for cleaning and removal.
Although the present invention is directed primarily to preventing or inhibiting the deposition of calcium carbonate and calcium phosphate scale, it is also applicable to inhibiting the deposition of other types of alkaline earth metal scales, especially those associated with calcium carbonate scale under the severe conditions described herein. For example, most industrial and commercial water contains alkaline earth metal cations, such as calcium and magnesium, etc., and several anions such as bicarbonate, carbonate, and phosphate. When combinations of these anions and cations are present in concentrations which exceed the solubility of their reaction products, precipitates form until their product solubility concentrations are no longer exceeded. These precipitates are alkaline earth metal scales. Thus, by alkaline earth metal scales is meant scales including but not limited to calcium carbonate, calcium phosphate and magnesium carbonate. These scales form frequently in the tubes of heat exchangers and on other heat exchange surfaces, such as those in cooling towers. Particular systems or applications areas where severe conditions lead to exceptional buildup of calcium carbonate and calcium phosphate scales, in addition to cycled up cooling towers, include reverse osmosis systems, sugar refining evaporators and certain types of gas scrubbers.
The synergistic combination of the present invention is used in amounts as threshold inhibitors to achieve calcium scale inhibition, rather than sequestering or chelating agents, although the combination of the present invention has dispersant properties as well and significantly reduces the adherency of any scale deposit which is formed, facilitating its easy removal.
Scale-forming compounds can be prevented from precipitating by inactivating their cations with chelating or sequestering agents, so that the solubility of their reaction products is not exceeded. Generally, this requires many times as much chelating or sequestering agent as cation, since chelation is a stoichiometric reaction; these amounts are not always desirable or economical. However, several decades ago, it was discovered that certain inorganic polyphosphates would prevent such precipitation when added in amounts far less than the concentrations needed for sequestering or chelating.
When a precipitation inhibitor is present in a potentially scale-forming aqueous system at a markedly lower concentration than that required for sequestering the scale-forming cation (stoichiometric), it is said to be present in "threshold" amounts. See, for example, Hatch and Rice, Indust. Eng. Chem. 31, 51-53 (1939); Reitemeier and Buehrer, J. Phys. Chem., 44(5), 535-536 (1940); Fink and Richardson, U.S. Pat. No. 2,358,222; and Hatch, U.S. Pat. No. 2,539,305.
Similarly, anionic and cationic polymers can be used as dispersants in accordance with methods known in the art, but the dosage levels necessary to achieve dispersion are in the range of 0.5-1.0% by weight of the aqueous system being treated, which is many orders of magnitude higher than the dosage levels used for the combination of the present invention. Thus, it is a unique aspect of the present invention that it is possible to achieve essentially non-adherent scale using only threshold inhibitor dosage levels of the synergistic combinations of the present invention.
Recently, attention has been focused on controlling scaling under severe conditions, where conventional treatments such as those described above do not provide complete scale control for calcium carbonate and calcium phosphate. Current technology in scale control can be used to inhibit CaCO.sub.3 scale up to 100 to 120 times calcite saturation, i.e., a water containing Ca.sup.2+ and CO.sub.3.sup.2- present at 100 times (100.times.) the solubility limit of calcium as calcite (calcite is the most common crystalline form of calcium carbonate). However, what is desired are inhibitors effective in greater than 100.times. water, where the calcite ions can be prevented from precipitating as calcium carbonate scale and also wherein the inhibitors are effective in inhibiting the formation of calcium phosphate scale as well, using substoichiometric amounts of an inhibitor. Further, the synergistic combinations of the present invention are especially useful under severe conditions characterized by a calcite saturation level of greater than 150.times. as defined in the paragraph immediately below, for the control of both calcium carbonate and calcium phosphate scales.
Severity of the scaling tendency of a water sample is measured using the saturation index, which may be derived in accordance with the following equation: ##EQU1## where SI is the saturation index for calcium carbonate, [Ca.sup.2+ ] is the concentration of free calcium ions, [CO.sub.3.sup.2- ] is the concentration of free carbonate ions, [CO.sub.3.sup.2- ] is the concentration of free carbonate ions, and K.sub.sp CaCO.sub.3 is the conditional solubility product constant for CaCO.sub.3. All of the quantities on the right side of the above equation are adjusted for pH, temperature and ionic strength.
Calculation and use of the saturation index, and generation of the data from which it is derived, are matters within the skill of the art. See, for example, Critical Stability Constants, Vol. 4: "Inorganic Complexes", Smith & Mantell (1976), Plenum Press; and Aquatic Chemistry, Chap. 5, 2nd ed., Stumm & Morgan (1981), Wiley & Sons.
Another characteristic feature of the severe conditions under which the scale controlling methods of the present invention are especially useful is high pH, greater than about 8.5. A related feature of such severe conditions is high alkalinity.
One of the particular advantages of the scale inhibiting combinations of the present invention is the exceptional calcium tolerances which they exhibit. Calcium tolerance is a measure of a chemical compound's ability to remain soluble in the presence of calcium ions [Ca.sup.2 +]. One of the parameters of scale control under severe conditions is pH. As pH increases, calcium tolerance decreases rapidly for traditional CaCO.sub.3 threshold inhibitors, e.g., 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) and amino tri(methylene phosphonic acid) (AMP). These inhibitors precipitate with calcium at alkaline pH's, rendering them useless as threshold scale inhibitors.
Early efforts to reduce scale formation in water-carrying systems employed compounds such as tannins, modified lignins, algins, and other similar materials. Chelating or sequestering agents have also been employed to prevent precipitation or crystallization of scale-forming calcium carbonate. Another type of agent which has been actively explored heretobefore as a calcium carbonate scale inhibiting material is the threshold active inhibitor. Such materials are effective as scale inhibitors in amounts considerably less than stoichiometrically required, and this amount, as already mentioned, is termed the threshold amount. Inorganic polyphosphates have long been used as such threshold active inhibitors. For examples of such materials, see Fink--U.S. Pat. No. 2,358,222; Hatch--U.S. Pat. No. 2,539,305; and Ralston--U.S. Pat. No. 3,434,969. Certain water soluble polymers, including groups derived from acrylamide and acrylic acid have been used to condition water containing scale-forming calcium carbonate. For example, See U.S. Pat. Nos. 2,783,200; 3,514,476; 2,980,610; 3,285,886; 3,463,730; 3,518,204; 3,928,196; 3,965,017; and 4,936,987. In particular, there has been employed anionic polyelectrolytes such as polyacrylates, polymaleic anhydrides, copolymers of acrylates and sulfonates, and polymers of sulfonated styrenes. See, for example, U.S. Pat. No. 4,640,793; 4,650,793; 4,650,591; 4,457,847; and 4,671,888. However, when used as threshold alkaline earth metal scale inhibitors, large dosages of these polymers are required, which in turn increases operating costs.
While polyether polyamino methylene phosphonates of the type that comprises one element of the synergistic combination of the instant invention, are known for their use for the control of alkaline earth metal scale having severe conditions wherein the pH is at least 8.5 and the calcite saturation is at least 150 times the solubility limit of calcium as calcite, U.S. Pat. Nos. 5,338,477 and 5,353,642, the aqueous system having the synergistic combination of the instant invention has not heretobefore been suggested. Further, as demonstrated herein, use of the polyether polyamino methylene phosphonates alone for the control of alkaline earth metal scales such as calcium carbonate and calcium phosphate simultaneously require large dosages making the use of the polyether polyamino methylene phosphonate alone expensive and therefore increasing operating costs to an unacceptable level.
In spite of this background material, there remains a very real and substantial need for a synergistic combination and a method of inhibiting the formation, deposition and adherence of scale forming salts in an aqueous system, such as for example, but not limited to, a cooling tower.